Preheat oven to 375 degrees F and place oven rack in middle. Spray an 8×8 baking dish with cooking spray and set aside.
In a large bowl, combine sugar, lemon juice, cinnamon, salt, and vanilla. Mix well, then add the pears and stir to coat. Next add the mangoes and stir to coat. Transfer the mixture to the prepared baking dish, spreading into an even layer.
In another bowl, mix the oat grahams, flour, brown sugar, and salt. Add in the melted butter and stir until mixed well and crumbly clumps have formed.
Sprinkle the topping over the fruit mixture evenly, then bake 40-45 minutes, until topping is lightly browned and fruit mixture is bubbling.
Remove from oven and allow to cool for at least 15 minutes. Serve topped with a scoop of vanilla ice cream, if desired.
Store in an airtight container in the fridge for 3-5 days. Leftovers can be eaten cold or warmed in the microwave or oven.